It works perfectly, but I'd like to specify a bucket to read and write from, but I'm not sure how to do that. I've seen examples online calling a GridFS constructor, but as you can see I'm not doing that here. The documentation also says that it's possible to supply a different bucket name, but I don't see anything on how.

How can I select which bucket that my files are saved to and read from?

gridfs-stream passes through the options object you pass a call to createWriteStream or createReadStream to the underlying mongodb driver to create a gridStore object to represent the file. The mongodb driver in turn recognizes root in the options object as an override of the default "fs" grid bucket prefix string.